i purchased a beautiful 2wd cab for my 91 4x4 4speed, and its ready for paint, and bed liner minus the 4x4 hump. i do have a bolt in one from an early/mid 70s truck that had a 4speed in it. can i cut the floor out and bolt that in? or do i need to cut the one peice out of the truck now and weld it in?

You should be able to lay the older tunnel over yours and draw a perimeter line as well as bolt holes on your 2wd cab floor. Then drill all the bolt holes and draw an inner perimeter line to cut out. Put 5/16 j-clips/nuts into the drilled holes and Use a non hardening caulk like 3m strip caulk to seal it. By using an non-hardening caulk it will seal as well as let you remove the tunnel if you need to do so.